In a copending application Ser. No. 85,821, filed Oct. 17, 1979 in the name of Norman L. Stauffer (now U.S. Pat. No. 4,309,063 issued Jan. 5, 1982), a low cost auto focus system was disclosed useful primarily on less expensive cameras; i.e., those with "between-the-lens" shutters. In that copending application, an additional lens, referred to as the focus lens, is utilized to produce the auto focus signal necessary for adjusting the position of the taking lens of the camera at the desired focus position. The focus lens is mechanically connected to the taking lens and moves along a parallel axis therewith and when the focus lens is at the proper focus position, as determined by the auto focus circuitry, the lens moving structure stops both the taking lens and the focus lens while the film is being exposed.
In a copending application Ser. No. 213,438 filed Dec. 5, 1980 in the name of Norman L. Stauffer, now U.S. Pat. No. 4,359,636, an improvement to this system is shown wherein moveable masking means are used to block a small portion of the radiation through the focus lens so as to assure that the detectors receiving the radiation have outputs which are as closely matched as possible to produce optimum focus results.
While the apparatus of the above-referred to copending applications operates to produce satisfactory results, it has been found difficult to make a small slave or focus lens system and still retain the focus performance required. For one thing, the size requirements dictate that a relatively small lens be used as a focus lens and thus the accuracy is not as great as would be desirable. Furthermore, the linkage required to couple the taking lens to the focus lens requires accurate adjustment and requires the use of linkage mechanisms which it would be desirable to eliminate.